with \(n\)  being the color index. It is not difficult to determine \(\Pi (p,q)\) 
in terms of the physical parameters of the particles involved into the decay
[1]}. By taking into account the ground states in the \(D\) 
and \(X_{0}\)  channels, we get
\(\Pi ^{\mathrm {Phys}}(p,q) &=&\frac{\langle 0|J^{D}|D\left( p\right) \rangle }{p^{2}-m_{D}^{2}}\langle D\left( p\right) K(q)|X_{0}(p^{\prime })\rangle \\&&\times \frac{\langle X_{0}(p^{\prime })|J^{\dagger }|0\rangle }{p^{\prime 2}-m^{2}}+\cdots , \) 
